WASHINGTON — One by one, Senator John McCain’s fellow prisoners from the Vietnam War have been dying, and at 78, he said, he is aware more than ever that his time will come, too. “Every single day,” he mused in an interview, “is a day less that I am going to be able to serve in the Senate.”

But no one should think that this reminder of his own mortality has tamed Mr. McCain, the bellicose Arizona Republican. If he had his way, the United States would have ground troops in Syria, more troops in Iraq and Afghanistan, and a steady supply of arms going to Ukraine.

Now, six years after losing the presidential election to Barack Obama, Mr. McCain finally has the only job in Washington, other than being president, that he ever wanted: chairman of the Senate Armed Services Committee, overseer of the American military and the nation’s defense policy. The question is whether he will use his new clout (and ability to subpoena) to make war or some accommodation with the White House.

“McCain now has the power either to destroy the president’s national security policy or shape it constructively,” said Leslie H. Gelb, president emeritus of the Council on Foreign Relations. “This is the first step to see whether he is going to use his new power to clobber Obama as he has for the past six years, or whether he will use it to try to shape and improve Obama’s policy.”

For now, despite hints that he is trying to reinvent himself from cantankerous Obama critic to elder statesman, Mr. McCain still seems to be in clobber mode. In an early assertion of his new authority, he said on Tuesday that he would push legislation to block Mr. Obama from sending detainees at the Guantánamo Bay military prison to Yemen.

He said in an interview last Thursday that Mr. Obama’s decision not to send more American troops to Iraq to thwart the Islamic State had put America at risk. “That attack you saw in Paris? You’ll see an attack in the United States,” Mr. McCain said. He repeated his frequent assessment that the president’s foreign policy is “a disaster” and “delusional.” He said “of course” he would have made a better commander in chief.

And he is still seething about his last visit to the Oval Office, in September 2013, when he and Senator Lindsey Graham, Republican of South Carolina, backed Mr. Obama’s plan to intervene militarily in Syria — only to watch the president change his mind.

“Didn’t even get the courtesy of a phone call,” Mr. McCain said. “When somebody looks you in the eye in the Oval Office and says they’re going to do something, don’t you take their word for it? I did. I took his word for it. And obviously I shouldn’t have.”

In short, he said of Mr. Obama’s foreign policy: “I’ve been right. He’s been wrong.”

For his debut as chairman, Mr. McCain is planning a series of hearings on national security strategy with a bipartisan cast of luminaries from administrations long past, among them former Secretary of State Henry A. Kissinger, 91, and two former national security advisers: Brent Scowcroft, 89, and Zbigniew Brzezinski, 86.

Denis McDonough, the White House chief of staff, said the president sees Mr. McCain as possessing “a very important viewpoint to consult with,” even if Mr. Obama rejects Mr. McCain’s interventionist stance. Mr. McDonough said Mr. McCain had influenced the Obama administration’s foreign policy, although when he was asked how, he paused.

Senior Senate Democrats say they respect Mr. McCain for his expertise and bipartisan deal-making on issues like immigration, and they hold out the possibility that Mr. McCain will do more on the Armed Services Committee than browbeat Mr. Obama’s nominees during confirmation hearings, as he has in the past. There are some signs of change: Mr. McCain is an enthusiastic supporter of Mr. Obama’s nominee for defense secretary, Ashton B. Carter, who is to appear before the committee in February.

“I sense he has found the place where his particular talents are most important and most useful to the country,” said Senator Jack Reed of Rhode Island, the senior Democrat on the committee. “He’s no longer on the quest to be president, he’s not being distracted by a steppingstone to the next thing, and he feels he’s taking over at a crucial time to shape the policy of the United States.”

Senator Charles E. Schumer, Democrat of New York, said Mr. McCain had an important role. “People know his personality, and they know how passionately he feels, and they know he sometimes uses hyperbole,” Mr. Schumer said. “But it’s all very sincere and very heartfelt and based on a whole lot of experience. And I think in a world of terrorism, even when people don’t agree with him, they’re sort of glad he’s there.”

For Mr. McCain, a former Navy pilot and the son and grandson of admirals, running Armed Services is the “fulfillment of a lifelong political aspiration,” said Mark Salter, a longtime adviser to Mr. McCain and co-author of several of the senator’s books.

Mr. McCain, in a sense, grew up on the Armed Services Committee. In 1977, four years after his release as a prisoner of war in Hanoi, the Navy assigned Captain McCain to be its Senate liaison. He traveled around the world with committee members like Gary Hart, a Democrat who later ran for president, and William S. Cohen, a Republican who became defense secretary under President Bill Clinton. John G. Tower, the committee chairman and a hawkish Texas Republican, became like a second father to Mr. McCain.

“It was Tower who inspired him to run for office and get into politics and get out of the military,” said Rick Davis, Mr. McCain’s longtime strategist. “So to have the job that Tower had — that’s a big deal to him. That’s full circle.”

Mr. Tower, who died in 1991, still inspires Mr. McCain today. Of his plan to hold hearings to explore America’s place in the world, Mr. McCain said, “That’s a page out of Tower’s book.”

As chairman, Mr. McCain has said, he will push to end the across-the-board budget cuts for the Pentagon known as sequestration, and he wants changes made to cut waste in the military’s procurement system, which has the defense industry on edge. And by using his new platform to call attention to global threats, he hopes to create a favorable climate for a Republican to win the White House in 2016. On Tuesday, Mitt Romney, who is considering another run, called Mr. McCain for advice.

“A year ago, before the beheadings, national security was a very low priority for the American people,” Mr. McCain said. “Now it’s a very high priority, and that was what Reagan did when he defeated Jimmy Carter. National security, peace through strength, became a very important phrase.”

Having tangled with Republican budget hawks (he once called Senator Ted Cruz of Texas and Senator Rand Paul of Kentucky “wacko birds”), Mr. McCain is buoyed by the recent election of several military veterans: Tom Cotton of Arkansas, Dan Sullivan of Alaska and Joni Ernst of Iowa. He has maneuvered to put them all on his committee, in hopes of bolstering what he calls “the internationalist wing.”

If Mr. McCain runs for re-election in 2016, as he says he is likely to do, he expects a Tea Party challenge. If he wins a sixth term, Mr. McCain will be 86 at the end of it, so friends say that race will probably be his last.

“I think John sees the next six years, really the next eight years, as the chance to close it out strong,” said Jack Keane, a retired Army general and close friend to Mr. McCain. Mr. Graham, who is Mr. McCain’s closest friend in the Senate, said that Mr. McCain is “looking at his legacy” and is hopeful about working with Democrats to reform business practices at the Pentagon.

“He wants to be the guy who did the hard things,” Mr. Graham said.

Mr. McCain is not particularly interested in talking about legacy. But he did say that he has been hit hard by the deaths of fellow war prisoners, men he considered brothers. So he is feeling a new sense of urgency, he said, to “jam-pack” his schedule and make the most of his time.

He said he had already decided on an epitaph for his tombstone: “He served his country."

A version of this article appears in print on , on Page A14 of the New York edition with the headline: At 78, an Untamed McCain Savors a New Dream Job in the Senate. Order Reprints | Today’s Paper | Subscribe